1. Technical Field
The present invention relates to disk drives for computers, and more particularly, relates to a power saving apparatus for hard disk drives used in such computers and a power management method for enabling a hard disk cache to operate in a write back mode and restricting access to hard disk drive in order to reduce battery power consumption.
2. Related Art
Most computers in use today includes a hard disk drive which is a random access memory device containing a magnetic head, a motor and one or more disks that store information. The motor turns a disk underneath the magnetic head. The disk contains electrically encoded data that is detected by the magnetic head as the head passes over the disk. The disk can be read from during a read operation or written to during a write operation of the hard disk drive as the magnetic head is moved along a radius of the disk and the disk spins under the magnetic disk.
Unfortunately, constantly spinning the disk in the hard disk drive consumes a large amount of power. Specifically, a motor must be devoted to spinning the disk to allow access to the full physical array of data on the disk. Powering a mechanical device such as a motor consumes a significant amount of power relative to the power consumed by the electronic circuitry within a computer. Consequently, it has become highly desirable to reduce the hard disk power consumption in order to maximize the energy efficiency, particularly when the computers use a rechargeable battery as a power source.
One power management technique commonly used in portable computers is turning off the hard disk drive motor when the hard disk drive has not been used recently. There are several methods of turning off the hard disk drive motor while leaving the remainder of the computer circuitry on. Many computers include a "sleep" button for allowing the user to power down the hard disk drive motor without powering down the entire computer in order to conserve battery power that would otherwise be wasted when the hard disk drive is not being accessed. Other methods of powering down the hard disk drive include programming a Basic Input/Output System (BIOS) driver to power down the hard disk drive after the passage of a predetermined time period during which the hard disk drive has not been accessed. Other adaptive power management techniques for hard disk drives are disclosed, for example, in U.S. Pat. No. 5,574,920 for Method For Controlling Power Down Of A Hard Disk Drive In A Computer issued to Parry, U.S. Pat. No. 5,535,400 for SCSI Disk Drive Power Down Apparatus issued to Belmont, U.S. Pat. No. 5,517,649 for Adaptive Power Management For Hard Disk Drives issued to McLean, U.S. Pat. No. 5,493,670 for Adaptive Disk SPN-Down Method For Managing The Power Distributed To A Disk Drive In A Laptop Computer issued to Douglis et al., U.S. Pat. No. 5,481,733 for Method For Managing The Power Distributed To A Disk Drive In A Laptop Computer issued to Douglis et al., U.S. Pat. No. 5,452,277 for Adaptive System For Optimizing Disk Drive Power Consumption issued to Bajorek et al., and U.S. Pat. No. 5,408,369 for Power Saving System For Rotating Disk Data Storing Apparatus issued to Miura et al. However, none of the conventional power management techniques addresses the balance between reduced power consumption and prevention of loss data to the sudden system power off.
Recent advances in portable computers in which an operating voltage of integrated devices is reduced from 5Volts to less than 3Volts use advanced power management (APM) system to facilitate suspending and resuming the system in order to minimize power consumption during its use. One example of such an APM system is disclosed in U.S. Pat. No. 5,560,023 for Automatic Backup System For Advanced Power Management issued to Crump et al., in which an automatic backup suspend system is also provided to benefit from the APM but with greater reliability.
Generally, the APM system has the following features: First, it lowers a system clock frequency when a computer system is not in use for a predetermined time period after access has been made to the computer system. Secondly, it automatically shuts off power supply to a display monitor when there is no activity in a keyboard or mouse for a predetermined time period. Finally, it suspends motor drive operation in hard disk drive if no access has been made to the hard disk drive for a predetermined time period. This power management scheme is based on shutting off power supply directed to the energy consuming parts or peripheral of the computer system, if the computer system is inactive for a predetermined time period. The power saving feature is particularly useful to battery powered computer systems using LCD display panels such as laptop and notebook computers, where most of the power consumption is attributed to hard disk drive operation. When the APM system is implemented for the hard disk drive in such portable computers, however, there is a frequent occurrence of the on/off of the driving motor of hard disk drive because of disk buffer cache operation provided between the hard disk and the system main memory.
Conventionally, the disk cache adapted its control algorithm to the well known write through mode to maintain cache coherency and to prevent loss of data at the sudden system power off. The write through mode cache operates such that every time there is a change of data in the cache, the APM system operates the hard disk drive to write the change into the hard disk. Even when the hard disk drive is in the power saving (i.e., standby) mode which turns off the hard disk drive motor, if data in the cache has changed, the motor is still driven to perform the write operation for the changed data. It has been my observation that the frequent power on/off of the hard disk drive motor will cause excessive battery consumption, and under some circumstance it will lead to the worst case that the power saving cannot be realized at all. Furthermore, when the motor is driven, there is a waiting period for the motor to arrive at the normal revolution speed, and this waiting period will lower the system performance.